heimdal: backport upstream commit to fix parallel build

Thanks to Vicente for an initial patch, and to Gustavo for pointing
out the relevant upstream fix.

Fixes:

  http://autobuild.buildroot.net/results/3a4/3a42c5b294a9a8602844ed1127ec5eadb60a6bed/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>